Well there are a lot of ways to look at this. McCann does hit 5th for the Braves, but this isn't at all their preference, it's only b/c Francoeur is hitting so ridiculously bad right now. So once he starts getting a bit better, or someone else does, they will probably move McCann out of the 5th slot (keep in mind, he had to hit like .330 to earn the chance to hit there, and he isn't likely to keep that up).
He is a great hitter, esp. when you look at his plate discipline numbers. He's not a ludicrous walker, but he doesn't K all that much. The power is legit (catcher standards) and if anything he has shown slightly less power for the past few weeks than he actually has.
Barrett is older, but I heard some rumbles in the offseason about how he was working out pretty hard. Also, looking at Ao:Go ratios, it seems clear that he's swinging for the fences more and becoming more of a power hitter.
All told, I think McCann ends the year in the .280 range w/ maybe 12-15 homers, but Barrett could be around the same average and run it up to about 20.